1. Field of the Invention
This invention relates to a ball screw integrated type linear movement guiding unit preferably suited for use in an operating arm of a robot, an X-Y table and so on, and more particularly to a low-cost ball screw integrated type linear movement guiding unit in which a ball return passageway constituting a ball circulating system is improved to provide a smooth operability even for a high-load and high-rigidity application.
2. Prior Art
One conventional ball screw integrated type linear movement guiding unit is disclosed, for example, in Japanese Utility Model Laid-Open Application No. 2-12554.
This conventional construction comprises a male-threaded shaft having a spirally extending ball screw groove in an outer surface thereof, and a guide rail embracing the male-threaded screw shaft in parallel relation thereto, the guide rail having axial ball-rolling grooves in its side surfaces. A nut is threadedly engaged with on the male-threaded screw shaft via a number of balls rolling through the groove, and ball-rolling grooves opposed respectively to the ball-rolling grooves of the guide rail are formed respectively in the opposite side surfaces of the nut. Through the rolling movement of a number of balls in the opposed ball-rolling grooves, the nut is guided to be linearly axially moved in the forward/rearward direction.
As a ball-circulating passageway of the ball screw system which guides and circulates the balls rolling in the ball screw groove of the ball screw, there is provided a ball-circulating tube of a U-shape incorporated in the upper portion of the nut. On the other hand, as a ball-circulating passageway of a linear movement guiding system which guides and circulate the balls rolling in the opposed ball-rolling grooves of the guide rail and the nut, there are provided through holes which are slightly larger in diameter than the ball and are formed in a thickened portion of the nut, the through hole extending parallel to the ball-rolling grooves. Curved passageways of a semi-doughnut shape each interconnecting the through hole and the ball-rolling grooves are formed in each of end caps mounted respectively on the front and rear sides of the nut.
However, in the above conventional ball screw integrated type linear movement guiding unit, in order to form the ball-circulating passageway of the linear movement guiding system for guiding and circulating the balls rolling in the opposed ball-rolling grooves of the guide rail and the nut, the axial through hole is drilled in the thickened portion of the nut, and the balls are passed directly through this hole serving as a ball return passageway. Therefore, because of the roughness of the drilled hole surface, scales formed on the drilled hole surface at a subsequent heat treatment of the nut, a step on a joined portion of the hole, and so on, a smooth passage of the balls is prevented, which results in a problem that the circulation of the balls becomes unstable.
Further, even if it is intended to increase the ball load passageway of the linear movement guiding system for a high-load and high-rigidity application, the length of the nut in its axial direction is increased, and therefore the length of the ball return passageway must also be increased. As a result, the hole having a high length-diameter ratio must be formed, and therefore there has been encountered a problem that the length is limited in this respect.
In order to overcome the difficulty with the working of the ball return passageway of the ball system integrated type linear movement guiding unit, the Applicant of the present application has earlier made a proposal in which the diameter of the through hole is made considerably larger than the diameter of the ball so as to improve the precision and efficiency of the working, and a pipe is press-fitted into this through hole to form the ball return passageway (U.S. Pat. No. 4,648,726).